In order to investigate how energy expenditure changes with over and underfeeding the following studies are underway. In one study, after careful calibration of weight maintenance EE, individuals undergo a series of measurements of 24 hour EE in a respiratory chamber in which they are fasting or overfed (by 200% of weight maintenance needs) a series of diets that vary in macronutrient content. This is to further investigate whether low protein diets may improve the detection of recruitment of adaptive thermogenesis. In addition, behavioral, metabolic and hormonal tests are performed to examine associated characteristics and to investigate the mechanism of the changes in EE. These individuals will also be followed up long term to look at what factors predict weigh change. Results from the first 15 individuals to complete this study have found that the change in energy expenditure with fasting and with overfeeding is reproducible. In addition, the increase in energy expenditure with overfeeding was highest in those overfed a normal protein high fat or high carbohydrate diet. Energy expenditure did not increase with low protein diets. Based on the difference in energy expenditure measured during fasting and overfeeding, the thermic effect of food (TEF) can be calculated. We have also found that TEF is inversely associated with adiposity. This study has also been investigating whether brown fat, known to increase thermogenesis with overfeeding in rodents, can be visualized with overfeeding in humans. In another study, after measurement of EE with over and underfeeding, and also after undergoing a series of metabolic and behavioral testing (including biopsies of muscle and fat), individuals will be admitted for 6 weeks of an inpatient dietary protocol involving underfeeding (for overweight and obese individuals) or overfeeding (for lean, obesity resistant individuals). During the inpatient study, all aspects of food intake, energy expenditure, and energy loss will be carefully measured to determine if differences in weight gain or loss can be attributed to recruitment of adaptive thermogenesis or other factors. Outpatient follow-up will continue after the inpatient phase to look at free living predictors of weight change as well. Recruitment for both studies is ongoing.